Ultrasonic transducers transmit and receive ultrasonic waves, and thus they are used for diagnosis of tumors etc. in human bodies, nondestructive tests of structures, and so forth. As the ultrasonic transducers, those utilizing vibration of piezoelectric substances have so far been used. However, with progress of the MEMS technology in recent years, capacitive micromachined ultrasonic transducers (CMUTs) comprising a vibration part formed on a silicon substrate have been actively developed aiming at practical use thereof. For example, U.S. Pat. No. 6,320,239 B1 (Patent document 1) discloses a single CMUT and a CMUT array.
In contrast to the conventional transducers utilizing piezoelectric substances, CMUTs have advantages of the wide frequency band of usable ultrasonic waves, high sensitivity, and so forth. Moreover, since they are produced by using LSI processing techniques, micro processing can be used. In particular, CMUTs is considered to be indispensable when ultrasonic devices are arranged in an array and independently controlled. This is because, while wiring for each device is needed and thus the number of the wiring in the array becomes a huge number, CMUTs are produced by using LSI processing techniques, and therefore the wiring is easy. Moreover, that is also because the CMUTS can be embedded on one chip of a circuit which processes signals from the ultrasonic transmission and reception part.
With reference to FIG. 1, fundamental structure and operation of CMUT will be explained below. Above a lower electrode 101, a hollow part 102 surrounded by an insulating film 103 is provided. Above the hollow part 102, an upper electrode 104 is disposed through the insulating film 103. If a direct voltage and an alternate voltage are superimposingly applied between the upper electrode 104 and the lower electrode 101, an electrostatic force is generated between the upper electrode 104 and the lower electrode 101, and a membrane 105 constituted by the insulating film 103 and the upper electrodes 104 above the hollow part 102 vibrates at the frequency of the applied alternate voltage to emit ultrasonic waves. Conversely, in the case of reception, the membrane 105 is vibrated by pressure of ultrasonic waves that arrive at the surface of the membrane 105. Then, the distance between the upper electrode 104 and the lower electrode 101 varies, and therefore ultrasonic waves can be detected as change of electrostatic capacitance. According to the aforementioned principle of operation, by applying a direct voltage between the upper electrode 104 and the lower electrode 101, an electrostatic force is generated between both the electrodes, and the membrane is deformed and stabilized at a deformation amount at which the spring restoring forth induced by the deformation and the electrostatic force are balanced.
CMUT is usually driven at such a direct voltage that the electrostatic force between the electrodes and the spring restoring forth are balanced. However, if a direct voltage larger than such a voltage that deformation amount of the membrane reaches about ⅓ of the distance between the electrodes, called collapse voltage, is applied, the electrostatic force between the electrodes becomes larger than the spring restoring force of the membrane, thus the membrane cannot be stabilized at a fixed position, and the surface 106 above the hollow part contacts with the surface 107 below the hollow part. If they contact with each other, there is produced a structure that the insulating film 103 is held between the upper electrode and the lower electrode without the hollow part. As a result, electric field intensity in the insulating film becomes large in the contacting region, and electrical charge is injected into the insulating film from the electrodes to generate fixed electrical charge in the insulating film. After the direct voltage is shut off, if a direct voltage is applied again between the two electrodes, the electric field between the electrodes is shielded by the fixed electrical charge in the insulating film, and the voltage used for optimal operation of CMUT fluctuates. Moreover, it is also possible that the insulating property of the insulating film may be degraded depending on the injection rate of electrical charge into the insulating film, and thereby the upper electrode and the lower electrode may be short-circuited. Therefore, the CMUT disclosed in Patent document 1 is usually used with a voltage significantly lower than the collapse voltage in order to prevent the surface above the hollow part from contacting with the surface below the hollow part.
Japanese Patent Unexamined Publication (KOKAI) No. 2009-272824 (Patent document 2) discloses a structure in which a floating electrode is embedded in an insulating film between electrodes in order to reduce the direct voltage for driving CMUT or make continuous application thereof unnecessary.